An alternative method is to simply leave the print face downwards in the developing bath, when the gelatine will flow from the surface as it is loosened by the hot water, and the print will develop automatically. It can be examined from time to time and its progress seen.
A correctly exposed print should be developed in four or five minutes.
Like all other photographic prints, it appears rather darker after drying than while wet, and allowance for this must be made in developing.
Although the image is sufficiently tough to bear the hot treatment without injury, it must on no account be touched by the fingers or by coming into contact with the edges of the dish or other prints. It is very soft, and it is impossible to touch it without removing some portion of the gelatine, and so leaving a mark. This is a sufficiently good reason for the advice given to develop prints one at a time, when there is practically no risk of injury if reasonable care is exercised.
It will be recognised that there is no sharp dividing line between gelatine that has remained soluble and that that has been rendered insoluble, excepting on the surface ; in the thickness of the film it is a gradual change from one condition to the other. Part of that remaining to form the image could be dissolved away if required.
When one print has been placed in cold water to rinse, the water in the developing bath should be raised to the initial temperature again and a second print commenced, the work being repeated until all are finished.
In the meantime, the print first developed is given a second change of water, and then placed in a dish containing some of the alum solution previously given. With care, and by using plenty of solution, several prints may be put in one dish together, but they must not be allowed to touch each other while in the solution or in being removed.
Prints should remain in the alum solution for about five minutes, then be rinsed in three or four changes of water, and hung up to dry. Small wooden clips form the most convenient method, as they can be readily suspended from a line. The prints must be allowed to dry spontaneously, and on no account must the surface be touched until they are perfectly dry.
It has been pointed out that when the film has been exposed under a negative the action of the light has been to render a certain part near the face practically insoluble, and that the condition does not change suddenly, but gradually merges into perfect solubility towards the back. It necessarily follows that there is a certain part of the film that is partially soluble only, and instead of dissolving rapidly like the perfectly soluble back, will only dissolve by prolonged treatment; and, consequently, long development will give a lighter print than shorter, as more of this semi-soluble gelatine will be dissolved.
This fact provides a very simple means of correcting slight over or under exposure of the print. If the print has been exposed too long a time in the frame and normal development would leave it somewhat dark, longer treatment in the developing bath will so far reduce it that it will be indistinguishable from one correctly exposed.
If the exposure has been too short the print should be removed from the bath as quickly as possible and placed in cold water. Development should be continued in water considerably cooler than usual—as cool, in fact, as will produce any action. Should this fail to remove sufficient of the gelatine the temperature of the water may be gradually raised until the print is sufficiently light.
If the print is much under-exposed there is no possibility of obtaining a satisfactory result.
If it is much over-exposed, the longer development will not reduce it sufficiently, as a point soon arrives when the degree of insolubility is too great for the normal temperature to soften. By raising the temperature, however, greater solvent power is exercised, and under all ordinary circumstances this should prove a sufficient remedy and ensure good prints. It is not desirable to raise the temperature beyond 120 or 125 degrees at most, and this should only be resorted to when a lesser degree of warmth—115, for example—has failed.
Development may be prolonged even in this hot water, but the risk of injury to the print is considerably increased.
This description has been kept as simple in character as possible in order to enable those commencing carbon printing to carry their first prints through the various stages successfully without the necessity of having to study more than those points absolutely essential for preliminary work.
In order to become thoroughly familiar with the carbon process, however, and derive the full advantage of its many attractive features in their ordinary work, it will be necessary to give much more detailed attention to the various operations already described, as well as to others that are equally necessary if anything more than the most simple and elementary work is attempted.
Although there are no special difficulties to overcome, and nothing to render carbon printing beyond the powers of any careful worker, yet the mistake must not be made of assuming that the work is so simple and easy that there is practically nothing to learn. However simple any process may be, it requires learning by experience, and it is only by careful attention to detail at every stage that success can be assured. In the most simple work failure may easily result from want of acquaintance with essential points or want of care in working details.
To a greater extent than in most other processes does this apply to carbon. The absence of any visible image and the extremely delicate nature of the film during development render thoroughly systematic and careful work absolutely necessary throughout. But assuming the capability of working systematically and of giving careful attention to detail, there is no reason why any amateur photographer should not adopt the process that gives more perfect satisfaction as a means of printing from negatives than any other. The variety of effect and colour possible, combined with richness and delicacy, is unsurpassed.
The defects and failures that may arise in the work already described—single transfer printing—are not many, and a knowledge of their cause will enable the worker to take precautions to prevent their occurring.
Should the backing paper refuse to strip off easily after the print has remained a minute or more in the developing dish, it indicates that the tissue has become insoluble. This may be due to keeping it too long before printing, or keeping prints too long before development, or by overprinting. The cold water bath advised before development facilitates removal of the backing paper, and also prevents another cause of insolubility from arising, keeping prints drying too long after squeegeeing to the backing paper. If the tissue is in good condition and the prints not overexposed, from fifteen to thirty seconds should be sufficient time for the hot water to render the removal of the backing paper easy. And it is desirable to remove it as soon as possible after immersing the print, but force should never be used.
When difficulty arises in removing the backing paper, hotter water should be tried; the degree of heat necessary for this will not be too great for developing, as it indicates partial insolubility of the gelatine right through the film.
In extreme cases the gelatine film may peel off the transfer paper instead of the backing paper leaving the film. This can only be caused by the tissue having become thoroughly insoluble and useless.
The use of the safe-edge will be readily recognised from this description. By its aid a thin strip of tissue unacted on by light is preserved as a margin to the print, which will be perfectly soluble if the tissue is in good condition. This margin will, of necessity, be the most soluble portion of the film, and consequently, the backing paper will readily loosen itself from the transfer paper at the edges, so that it can be easily lifted and pulled away without any risk of bringing the film with it. Without the safe-edge successful removal of the backing paper would be impossible ; wherever a dark shadow formed the margin of the print, the small degree of solubility of any portion of the film at that point would induce it to cling to the backing paper and loosen its hold to the transfer.